I suspect that the slower speed is not related to RAM usage. Activity Monitor reports 93.2 MB for an empty project, which of course grows depending on the project being loaded. For a comparison, the average browser (Chrome, Firefox, etc.) can use more than 900 MB.

With AutoCAD projects, pay attention to hatches as they are often made of dotted or tiny patterns that can slow down the entire project. Simply turning off their layer can make a huge difference.